Many peptides are known which are biologically active and are useful in the treatment of diseases and which contain a disulfide ring. Calcitonins, which are useful in the treatment of Paget's disease, contain a ring structure involving cysteine groups at the 1st and 7th positions in their amino acid chains. Oxytocin is useful for the therapeutic induction or stimulation of labor in humans and animals and also to control pastpartum uterine bleeding. It contains a disulfide ring structure between the cysteine groups at positions 1 and 6 in its amino acid chain. Vasopressin and its analog lypressin are used as antidiuretic drugs in man and contain disulfide ring structures between the cysteine groups at positions 1 and 6 in their amino acid sequences (handbook of Biochemistry, pages C-164 to C-188).
Although the kind and sequence of the amino acid groups for the calcitonins, oxytocin, vasopressin, and other such naturally occurring peptides may vary depending upon the species from which they are obtained, all such peptides which were originally obtained from natural sources, such as by extraction from the glands of humans, domestic animals, fishes, frogs, or reptiles, contain the ring structure referred to above. The amino acid sequence of some known biologically active peptides containing cysteine groups joined by disulfide bonds in a ring structure are given in Table I.
TABLE I __________________________________________________________________________ Typical Peptides Containing Cysteine Ring Structures __________________________________________________________________________ Oxytocin: ##STR1## Vasopressin: ##STR2## Salmon Calcitonin: ##STR3## Human Calcitonin: ##STR4## Porcine Calcitonin: ##STR5## Bovine Calcitonin: ##STR6## __________________________________________________________________________
in prior attempts by others to prepare synthetically a peptide such as those referred to in Table I, the only method available for producing a closed disulfide ring structure was to attempt to form the intermediate peptide having the desired amino acid chain and then subject this peptide to an oxidative process using oxidizing agents, to form the disulfide bond between the two cysteine residues. Such oxidative methods have been described in the literature (Katsoyannis, P.G., The Chemistry of Polypeptides, Plenum Press, 1974, pages 60-85). A main disadvantage of these processes is the exposure of the highly labile peptide molecule to oxidizing agents. This treatment can cause inactivation of the peptide resulting in a lower yield of biologically active products.
The art has long needed satisfactory processes for the formation of the disulfide bond between the cysteine moieties of a peptide which do not require the use of oxidizing agents. Accordingly, we have set ourselves to the discovery of practical and efficient methods for the formation of a cyclic disulfide bond between the cysteine moieties of a peptide.